Fuel Balance Sys. 2005 GMC K3500 Cab & Chassis

Specific repair issues for GM RWD trucks, SUVs & vans
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oldsmobile39
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Fuel Balance Sys. 2005 GMC K3500 Cab & Chassis

Post by oldsmobile39 »

Good morning. Pulling our hair out for the past couple of days on this one. Cab & Chassis with a 6.6 Duramax LLY with dual tank setup and just figured out how the fuel system works. Now honestly- to our shop this is a beast of its own and we were not at all familiar with the fuel transfer system. After doing much research we found out the following: basically a high pressure pump in the valley is pulling fuel from the front tank and there is a fuel transfer pump up in the rear of the truck in the frame rail which transfers fuel from the rear tank to the front as needed to balance out the fuel level across the board as it were. There is a sending unit in each tank, and of course the fuel gauge on the dash with a LOW FUEL MIL. NOW- here's my problem. , I see the system works this way but on this truck it does not. In my customer's truck the rear tank is being drawn down while the front tank is staying full. Last week before dropping the truck off for the weekend he ran the rear fuel tank almost empty. The fuel gauge went to the bottom and the fuel light came on. Now- he put about $80 worth of fuel in the truck and the gauge still reads Empty and the LOW FUEL MIL is illuminated. I have checked the transfer pump and it it kinda rotted out but with my Solus PRO it can engage the relay which engages the pump sucessfully. The only problem with that test function is that the pump will not shut off unless I kick my Solus PRO out of the test and cycle the ignition key to reset everything. SO- if the system is setup to work a specific way how and I getting a "backwards" operational result from the fuel system. I can assume that one of the the sending units is bad (which incidentally I will be checking today), but why would the rear tank be the feed tank and the front tank stay full if the system is designed to draw from the front and use the rear as a supplement? Any assistance would be appreciated (especially from anyone who has run into this issue before).
I don't want to be the one the battles always choose, because inside I realize that I'm the one who's confused.
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ricmorin
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Re: Fuel Balance Sys. 2005 GMC K3500 Cab & Chassis

Post by ricmorin »

Likely the ill-reporting fuel senders are causing the rear tank to empty out. All bets are off on this system when the senders don't work right. Fix the senders first, then re-evaluate system operation.
Ric Morin - Volunteer Forum Moderator, Shop Owner, ASE Master L1
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